Dental plaque develops as a complex biofilm on the tooth surface and is a direct precursor Of periodontal diseases, one of the most common bacterial infections in developed countries. The general principles that cause the transformation of a commensal plaque biofilm into a potentially pathogenic entity are understood and involve attachment to the tooth surface by early colonizers, such as oral streptococci, which then provide an attachment substrate for the subsequent colonization by periodontal pathogens such as Porphyromonas gingivalis. Adherence between S. gordonii and P. gingivalis is thus considered to be an important factor that facilitates P. gingivalis colonization. Investigations in our laboratory have revealed that this adherence interaction is multimodal, involving several interacting adhesin and receptor molecules. Adhesins identified to date include the P. gingivalis fimbriae and an outer membrane protein of 35 kDa along With the S. gordonii Ssp proteins. Detailed analysis of the S. gordonii SSpB protein indicated that a C-terminal domain is responsible for the binding activity. The objectives of this proposal are: to further investigate the functionality of the SspB protein; to characterize the cognate binding receptor for the streptococcal Ssp proteins; and to investigate the S. gordonii-P. gingivalis co-adherence event in the context of a developing biofilm. The functional domain of the SspB protein will be explored by the construction of synthetic peptides and chimeric molecules comprising part Ssp protein and part PAc protein (a structurally related but functionally distinct molecule from S. mutans). These engineered molecules will be tested for adherence to P. gingivalis. The cognate receptor for the S. gordonii Ssp proteins will be identified and characterized by a variety of biochemical and molecular biological approaches. The contribution and role of the Ssp and fimbrial adhesins in S. gordonii-P. gingivalis co-adherence in the development of a dental plaque biofilm will be determined using genotypically defined strains and confocal scanning laser microscopy. The information provided by these studies will enhance our understanding Of how pathogenic and commensal plaque bacteria interact on a cellular and molecular level during the colonization process and biofilm development. Such insights will provide a knowledge base that will facilitate the development of novel methods to control periodontal disease based on inhibition of colonization mechanisms or interference with the regulatory mechanisms that control the expression of adhesins.